The roles of tryptophan, kynurenne, 3‐hydroxyanthranilic acid, and quinolinic acid as niacin precursors were investigated by assessing their germination and growth‐promoting effects in orchid embryo and seedling cultures. Only niacin, 3‐hydroxyanthranilic acid, quinolinic acid and kynurenine exhibited a growth‐promoting effect on seedlings of all ages. Tryptophan inhibited the growth of 70‐day‐old seedlings but enhanced growth of older seedlings. The data presented are interpreted to be consistent with the hypothesis that orchids may synthesize their niacin by a pathway similar to that of certain bacteria, fungi, birds, and mammals.